CQRS + Event Sourcing: (is it correct that) Commands are generally communicated point-to-point, while Domain Events are communicated through pub/sub?

Question

Didn't know how to shorten that title.

I'm basically trying to wrap my head around the concept of CQRS (http://en.wikipedia.org/wiki/Command-query_separation) and related concepts.

Although CQRS doesn't necessarily incorporate Messaging and Event Sourcing it seems to be a good combination (as can be seen with a lot of examples / blogposts combining these concepts )

Given a use-case for a state change for something (say to update a Question on SO), would you consider the following flow to be correct (as in best practice) ?

• The system issues an aggregate UpdateQuestionCommand which might be separated into a couple of smaller commands: UpdateQuestion which is targeted at the Question Aggregate Root, and UpdateUserAction(to count points, etc) targeted at the User Aggregate Root. These are send asynchronously using point-to-point messaging.

• The aggregate roots do their thing and if all goes well fire events QuestionUpdated and UserActionUpdated respectively, which contain state that is outsourced to an Event Store.. to be persisted yadayada, just to be complete, not really the point here.

• These events are also put on a pub/sub queue for broadcasting. Any subscriber (among which likely one or multiple Projectors which create the Read Views) are free to subscribe to these events.

The general question: Is it indeed best practice, that Commands are communicated Point-to-Point (i.e: The receiver is known) whereas events are broadcasted (I.e: the receiver(s) are unknown) ?

Assuming the above, what would be the advantage/ disadvantage of allowing Commands to be broadcasted through pub/sub instead of point-to-point?

For example: When broadcasting Commands while using Saga's (http://blog.jonathanoliver.com/2010/09/cqrs-sagas-with-event-sourcing-part-i-of-ii/) could be a problem, since the mediation role a Saga needs to play in case of failure of one of the aggregate roots is hindered, because the saga doesn't know which aggregate roots participate to begin with.

On the other hand, I see advantages (flexibility) when broadcasting commands would be allowed.

Any help in clearing my head is highly appreciated.

Answer 1

Yes, for Command or Query there is only one and exactly one receiver (thus you can still load balance), but for Events there could be zero or more receivers (subscribers)